Determining the success of spinal fusion surgery in a manner that maximizes accuracy while minimizing trauma to the patient remains a significant challenge for orthopaedic surgeons. While effective as diagnostic tools in many other settings, clinical and radiographic examinations including conventional radiography, magnetic resonance imaging (MRI), and computed tomography (CT) are limited in their ability to provide a clinically accurate assessment of fusion success [27, 68, 69, 70, 71, 72]. Although these imaging methods can determine whether there is bridging between implant and bone (i.e. structural stability), they cannot detect small motions of the implant and/or bone, motions that indicate whether the levels intended to be fused move as one unit and no longer as separate segments [71] The latter is a measure of the functional integrity of the fusion and is key to the long-term success of the surgery [71].

Direct surgical exploration is considered the most reliable method for accurately determining fusion success, both from a structural and functional perspective, following spinal fusion surgery [6, 7, 73] Unfortunately, this method is highly invasive, costly and seldom used [33-74]. Furthermore, surgical exploration does not provide any longitudinal information as to the stability of the implant, including the monitoring of any occurrence of migration [7, 68, 74]. In contrast, Roentgen Stereophotogrammetric Analysis [75] can detect the presence or absence of mobility between intervertebral segments (i.e. functional stability of the fusion) with a high degree of accuracy [28, 29, 68, 74, 76, 77, 78, 79]. RSA can capture even the most minutest motions that occur between implant and bone and can monitor changes in mobility over time [79]. Consequently, RSA has been deemed a superior measure of the progression of spinal fusion, as defined by decreasing intervertebral mobility [28, 29, 68, 74, 77, 78, 79, 80].